In wireless sensor networks all objects are provided with computing and communications functions to realize an environment where communications with the objects can be made at anytime and anywhere regardless of types of networks, devices and services. A sensor node in the wireless sensor networks obtains sensing information using a sensor and transmits the sensing information to external devices via the network, thus processing and managing the sensing information in real-time.
FIG. 1 illustrates a configuration view of conventional wireless sensor networks.
As shown in FIG. 1, the wireless sensor network includes: sensor fields 103, 104 and 105 respectively formed of sensor nodes 100, 101 and 102, each sensor node having a communications module and a sensor for recognizing objects or monitoring environmental conditions around the sensor; sinks 106, 107 and 108 for receiving sensing information collected in the sensor fields 103, 104 and 105, respectively; and a gateway 109 for routing the sensing information received from the sinks 106, 107 and 108 to transmit the sensing information to a management server 111 via a broadband communications network 110. The sensor nodes 100, 101 and 102 and the sinks 106, 107 and 108 can interoperate with existing infrastructures, such as satellite communications network, wireless LAN, Bluetooth, wired Internet and the like, via the gateway 109.
FIG. 2 illustrates a detailed configuration view of a sensor node shown in FIG. 1. In FIG. 2, among the sensor nodes 100, 101 and 102 of FIG. 1, only one sensor node, e.g., 100, is illustrated for the sake of simple illustration. However descriptions below can be also applicable to the remaining sensor nodes 101 and 102.
As shown in FIG. 2, the sensor node 100 includes: a program storage unit 201 for storing therein operating programs; a wireless communications unit 202; a control unit 203; a power supply source 204, which may be a battery, for supplying driving power to the sensor node 100; a sensing unit 205; and a signal processing unit 206 for processing sensing signals. In general, the sensor node 100 is operated by the limited power of the battery and thus, low power consumption design needs to be preferentially considered when configuring the sensor node 100. Accordingly, the sensor node 100 is generally formed of low power elements, e.g., 8-bit CPU, low power communications elements and peripheral circuits.
In addition to the hardware configuration of the sensor node 100, efficient usage of the limited power of the sensor node 100 is also directly coupled with a lifetime of the sensor network. It is known that the sensor node 100 consumes most of the limited power thereof when transmitting or receiving data via the wireless communications elements. Accordingly, the sensor node 100 periodically performs state transitions between an idle state and an active state. To be specific, operation systems, protocols and the like for use in the sensor network are designed to maintain most of operation time of the sensor node 100 in the idle state and allow the sensor node 100 to obtain sensing information and transmit the sensing information to the management server 111 via the sink 106 during the active state, which is relatively shorter period of time than the idle state. The periodical state transitions between the idle state and the active state corresponds to ON/OFF control of wireless communications elements in the sensor node 100 for transmitting and receiving the data.
As described above, due to the limited power of the sensor node 100, low power consumption elements are preferred as sensor elements for obtaining the sensing information in most applications, and, even a sensor network using the low power consumption sensing elements can sufficiently obtain simple sensing information such as temperature, humidity, illuminance and the like or detect an occurrence of a target event.
However, for example, in order to obtain the sensing information in various indoor and outdoor environments via the sensor network, signal processing through complicated operations, e.g., a fast Fourier transform (FFT), a discrete cosine transform (DCT) and the like, is needed for specific sensing information. In order to perform such signal processing, high power consumption sensing elements having high sensitivity and high performance are needed. But, the high power consumption sensing elements can be employed only when the sensor nodes are main-powered, when the battery basically provides high capacity of power or when the sensing elements in operation can be supplied additional power from an additional power supply source such as solar heat or the like.
In the sensor network, the signal processing for converting the sensing information to data for use in detecting or monitoring environmental conditions can be performed in a system having a power supply source capable of supplying higher capacity of power thereto and higher computing power compared to the sensor node 100, i.e., the signal processing can be performed in the sink 106 or the management server 111. Also, the signal processing can be performed through a distributed-processing in the sensor nodes 100. Below, signal processing methods in the sensor network will be described with reference to FIGS. 3 to 5.
FIG. 3 illustrates a configuration view of a conventional sensor network in which sensing signal processing is performed through a centralized-processing. FIG. 4 illustrates a configuration view of a sensor network in which sensing signal processing is performed through a groupwise distributed-processing. FIG. 5 illustrates a configuration view of a sensor network in which sensing signal processing is performed through a fully distributed-processing. In FIGS. 3 to 5, sensor nodes are distinguished from each other by using different reference numerals 301, 311 and 321, respectively, and sinks/gateways, each of which is formed by integrating a sink and a gateway in a single body, are distinguished from each other by using different reference numerals 305, 315 and 325, respectively.
Total power consumption of the entire sensor network increases in proportion to an increase in the number of information delivery hops and an amount of information. When a sensor node 301 includes an A/D and D/A converter 302 and a sensor 303 and a sink/gateway 305 includes a signal processing unit 304 as shown in FIG. 3, data obtained in the sensor node 301 is transmitted to the sink/gateway 305 via multiple hops (i.e., while passing through multiple nodes) without being subjected to additional processes and then processed through a centralized-processing in the sink/gateway 305. In this case, data transmission of a large amount of sensing information or duplicated data transmission thereof occurs in the sensor network, thereby causing high network traffic within the sensor network and increasing power consumption. Consequently, a lifetime of the sensor network is shortened.
Research has been conducted to develop methods to supplement the aforementioned defect of the centralized-processing system. In an effective one of the methods for reducing the necessity for the data transmission via multiple hops and duplicated data transmission, sensor nodes in a sensor network are grouped to form a number of clusters and sensing information of sensor nodes in a cluster are transmitted to a cluster head of the cluster to be subjected therein to signal processes, e.g., data aggregation, data fusion and the like, thereby increasing the lifetime of the network. For example, when a sensor node 311 includes an A/D and D/A converter 312 and a sensor 313 and a cluster head 316 is provided with a signal processing unit 314 as shown in FIG. 4, an excessive amount of data transmission or duplicated data transmission toward the sink/gateway 315 can be reduced, thereby reducing the total power consumption of the entire network. Another effective method will be described with reference to FIG. 5. When each sensor node 321 includes an A/D and D/A converter 322, a sensor 323 and a signal processing unit 324 as shown in FIG. 5, sensing signals of the sensor network are processed through a fully distributed-processing. Accordingly, traffic toward a sink/gateway 325, i.e., traffic in the network, can be further lowered compared to the case using the cluster head 316 as shown in FIG. 4.
However, even with the above described groupwise or fully distributed-processing method, a sensor network configuration capable of working for a longer period of time with lower power consumption and processing high-accuracy information is still required.